1. Field of the Invention
The present invention relates to a sheet guide device for guiding a sheet passing through a branch-off guide path that is branched off, and an image forming apparatus that includes the sheet guide device in an apparatus main body and causes the sheet on which an image has been formed to be guided by the sheet guide apparatus.
2. Related Background Art
In recent years, image forming apparatuses such as copying machines, printers, facsimiles, and all-in-one machines integrating their functions have become widely popular. These image forming apparatuses have been structured based on the copying machines. However, in association with the recent rapidly widespread use of personal computers, the image forming apparatuses structured based on the printers are now becoming mainstream. Also, it has become common practice that the image forming apparatuses employ duplex printing in which images are formed on both surfaces of the sheet from the viewpoints of environmental protection, resource savings, and the like. Thus, various models of the image forming apparatuses including a duplex image forming function are increasing in number.
In addition, the duplex image forming function of the image forming apparatus is being enhanced and the size thereof is being reduced. This trend leads to the necessity of sharply shifting a sheet transport direction of transporting the sheet within the apparatus main body. As a result, it has become essential to make smaller than ever the radius of curvature of the sheet guide path for guiding the sheet.
FIG. 11 is a schematic sectional view of a printer as an example of the conventional image forming apparatuses which is taken along a sheet transport direction.
In a printer 130 as shown in FIG. 11, a toner image is transferred onto a sheet P fed from a sheet feed portion 101 in an image forming portion 102. The sheet is then passed through a fixing device, so that the toner image is fixed to the sheet. Immediately after that, a transport path switch gate 111 is switched to thereby select the sheet transport direction. The subsequent transport paths are bent by approximately 90 degrees. Due to the switching of the transport path switch gate 111, the sheet is delivered to a face down tray (hereinafter, referred to as xe2x80x9cFD trayxe2x80x9d) 106 or selectively transported to a surface reverse transport path 119. The sheet whose surfaces have been reversed in the surface reverse transport path 119 is transported in a switch back manner in the surface reverse transport path 119, and then, a front surface and a back surface of the sheet are reversed. After that, the sheet is passed along a duplex transport path 120, and again sent to the image forming portion 102 and the like, so that another toner image is formed on the other surface. Accordingly, toner images are formed on both surfaces of the sheet. The sheet of which surfaces both have a toner image formed thereon is passed through the fixing device 103. Finally, the sheet is delivered to the FD tray 106.
Incidentally, in recent years, the cases where color printers are used to output high density color images have been increased in offices, homes, and the like regardless of special places. This is because the high density color images are often used as backgrounds (solid images as backgrounds) in preparing materials for presentations or the like, or because color photo images and the like are often outputted due to the widespread use of digital cameras.
Also, in the fixing devices of the color printers, compared with the fixing devices of conventional black-and-white printers, a larger amount of toner must be fused and fixed to the sheet. Therefore, for example, in the case where the fixing device is of a fixing roller type, a long period of time is necessary for the sheet to be passed through a fixing nip portion in the fixing device.
Accordingly, the sheet on which the toner image is formed in the color printer exits the fixing device with its surface remaining at a higher temperature than in the case where the toner image is formed in the conventional black-and-white printer. In general, the heat quantity supplied to the fixing device of the color printer is approximately 2 to 3 times larger than the fixing device of the black-and-white printer. In addition, in accordance with the downsizing of the printers, it is demanded to provide immediately after the fixing device a bend and transport path in which the direction of a sheet transport path is shifted by approximately 90 degrees in the transport path switch gate.
However, in the small-sized color printer under the above-mentioned circumstances, the sheet is bent and transported while a high density color image surface of the sheet is in contact with the transport path switch gate. Therefore, failure easily occurs in bending and transporting the sheet.
First, as a condition that the failure occurs most in bending and transporting the sheet, there is a case where, in the duplex printing in which toner images are formed on both surfaces of the sheet, a high density toner image is printed on the first surface. That is, the sheet in which the toner image has been transferred onto and fixed to the first surface is guided by the transport path switch gate 111 to be passed along the surface reverse transport path and the duplex transport path. When another toner image is then formed on the second surface and the sheet is passed through the fixing device 103 again, there is a case where the high density toner image on the reversed first surface which has been once fixed is fused by the heat of the fixing device 103. Immediately after that, before the toner of the fused toner image is completely cooled down to become hard, the sheet transport path is shifted upward by approximately 90 degrees in FIG. 11 by the transport path switch gate 111. Therefore, in some cases, due to the transport resistance applied when the sheet P is bent and transported upward, the self weight of the sheet P on which the high density image has been formed, and the like, the back surface of the sheet P is pressed against the transport path switch gate 111 and the toner surface of the back surface adheres to the transport path switch gate 111. This leads to the failure in transporting the sheet P.
In this case, in the periphery of a distal end portion of the sheet P, even if there is a slight adhesion resistance with respect to the transport path switch gate 111, the sheet can be transported by the transport force of a fixing roller pair 104 of the fixing device 103 or a fixing delivery roller pair 105. However, as shown in FIG. 12, in accordance with the sheet P being transported upward, the transport resistance of the sheet, the self weight of the sheet on which the high density image has been formed, and the like increases, thereby making it difficult to transport the sheet. The transport resistance of the sheet becomes largest immediately before the distal end of the sheet reaches a delivery sheet vertical pass roller 112 subsequent to the transport path switch gate 111. Thus, the force by which the toner surface of the back surface is pressed against the transport path switch gate 111 is also largest, so that there is a case where the adhesion of the sheet is caused and the failure in transporting the sheet occurs in the printer 130.
Second, as the failure that less often occurs than the above failure, there is a similar case where, in the duplex printing in which a toner image is formed on both surfaces of the sheet, when the sheet whose first surface having a high density image printed thereon is guided by the transport path switch gate 111 and transported to the surface reverse transport path 119, the adhesion of the sheet to the transport path switch gate 111 is caused as shown in FIG. 13. That is, there is a case where the high density image transferred onto and fixed to the first surface is fused by the heat of the fixing device 103. Immediately after that, before the toner of the fused toner image is completely cooled down to become hard, the sheet transport path is shifted downward by approximately 90 degrees in FIG. 13 by the transport path switch gate 111. Therefore, due to the self weight of the sheet on which the high density image has been formed and the like, the force is applied to the sheet in the direction for pulling the front surface thereof apart from the transport path switch gate 111. However, due to the transport force of the fixing device 103, the sheet is pressed against the gate surface of the transport path switch gate 111, so that there is a case where the toner surface of the front surface adheres to the gate member, which leads to the failure in transporting the sheet.
The present invention has an object to provide a sheet guide device for guiding the sheet smoothly without causing a phenomenon of sheet adhesion to a guide member, which is disposed in a branch-off portion of a branch-off sheet guide path, selects a guide path for the sheet, and guides the sheet and an image forming apparatus having the sheet guide device in an apparatus main body thereof which guides the sheet on which an image has been formed to the sheet guide device.
In order to achieve the above object, according to one aspect of the present invention, a sheet guide device includes:
a branch-off sheet guide path that is branched off from a sheet guide path that guides a sheet;
a guide member that is rotatably disposed in a branch-off portion of the branch-off sheet guide path and selectively guides the sheet to respective sheet guide paths that are branched off in the branch-off sheet guide path; and
a guide rotary member that is located in the branch-off portion and capable of rotating in a sheet transport direction, and
in the sheet guide device, a rotational surface of the guide rotary member is protruded from a sheet guide surface of the guide member which guides the sheet.
According to another aspect of the present invention, an image forming apparatus includes:
image forming means for forming a toner image on a sheet;
fixing means for fixing the toner image to the sheet on which the toner image has been formed by the image forming means;
a branch-off sheet guide path that is branched off from a sheet guide path that guides the sheet on a downstream side of the fixing means;
a guide member that is rotatably disposed in a branch-off portion of the branch-off sheet guide path and selectively guides the sheet to respective sheet guide paths that are branched off in the branch-off sheet guide path; and
a guide rotary member that is located in the branch-off portion and capable of rotating in a sheet transport direction, and
in the image forming apparatus, a rotational surface of the guide rotary member is protruded from a sheet guide surface of the guide member which guides the sheet.